Centralized alarm control panels, along with associated detection systems, are often provided in monitoring systems for large buildings or building complexes. One aspect of the functionality of the alarm control panel is to supervise each of a plurality of sensor units or detectors which are located throughout the building or the building complex.
The units are conventionally connected to the alarm control panel by two or more conductors. It has become a preferred practice to combine electrical power transmission and information transmission onto a single pair of conductors.
In addition, it has become desirable to form the sensor or detector units as two part devices. One part is a base which is surface mountable and which is hard-wired into the two wire cable of the monitoring system. A plug-in head or detector is arranged for being removably attachable to the base.
The use of plug-in sensor or detector units facilitates both installation and long term maintenance. One problem with the use of removable detector heads lies in the fact that they are removable. If a detector or a sensor head is removed, the alarm control panel loses the ability to sense combustion or intrusion in the area or region covered by the missing detector or sensor. In addition, a missing head could disrupt operation of the entire system.
As a result, it has been recognized that it is important that the alarm control panel be able to not only function properly where one or more of the sensor or detector heads is missing, but also to be able to detect the removal of the sensor or detector head. One arrangement for enabling the control panel to continue to work properly in the absence of one or more detectors uses a spring structure as disclosed in Spang et al., U.S. Pat. No. 4,829,283, assigned to the assignee of the present invention.
There continues to be a need for supervision devices which not only make possible the continued operation of an alarm system in response to a detector or sensor head being removed, but also provide a capability for detecting the fact that one or more sensor or detector units have been removed. Preferably, such a device would not increase the cost or complexity of the base assembly, but yet would provide the capability of informing the control panel that one or more detector or sensor heads had been removed.